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00702132154.l1DLsAfI042441@apollo.backplane.com> <1a00a5aa0702141513t42a9e2d1qf441bc1b74e66824@mail.gmail.com>
From: "Simon 'corecode' Schubert" <corecode@fs.ei.tum.de>
Subject: Re: Plans for 1.8+ (2.0?)
Date: Thu, 15 Feb 2007 01:49:34 +0100
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Chris Csanady wrote:
> Yes, I was considering it as a replacement for RAID-5. The idea being
> that, for a given filesystem block, you would divide it into
> sub-blocks and compute ECC blocks. These would then be distributed
> across the cluster nodes.
>=20
> For example, consider a 32kB filesystem block. Divide it into 4kB
> sub-blocks, and compute 3 4kB ECC blocks. Now, distribute those 11
> blocks over 11 separate nodes. Any three nodes can fail, plus space
> overhead is only 38% in this case. To provide the same guarantee with
> mirroring would carry a 300% overhead. While mirroring may be
> acceptable in terms of disk space, network I/O will likely be a
> problem.
How do you save on network IO there? You have to query 8(!) boxes to ret=
rieve one block. Okay, you might choose 8 out of 11, but that's still a =
lot. For writing, you of course have to write to all 11. If you go mirr=
oring, you can run the complete block from one source (you can of course =
also interleave with a mirror). For writing, you can use multicast/broad=
cast on LAN. That makes mirrored writes as efficient as normal writes. =
When you do ECC, you have to write all 138%. If you run over WAN, you wo=
n't be able to save with multicast probably, but then your block distribu=
tion will make it really hard to get a constant stream due to massive jit=
ter.
I'm not yet convinced :) Disk space is really cheap these days.
cheers
simon
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